import os
list_exist = []
files = os.listdir(r'E:\tmp\Themes\bk_image')
for i in files:
    jishu = i.split('.')[0]
    i = r'E:\tmp\Themes\bk_image' + '\\' + i
    new_name = r'E:\tmp\Themes\bk_image\\' + jishu + '.jpg'
    os.rename(i, new_name)